Alternators are commonly employed as the basic source of electrical power in automotive vehicles; the alternator output is rectified and is used to charge the vehicle battery and to operate a wide variety of electrical devices incorporated in the vehicle. The size and weight of the alternator should be held to a minimum; the cost of the alternator is also a prime consideration. Even more important, perhaps, is the necessity for the alternator to operate with little or no attention over long periods of time because vehicle operators, particularly the owners of private passenger automobiles, frequently neglect maintenance procedures for extended periods.
The service requirements imposed upon an automotive alternator can be quite severe. The load on the machine may vary over wide extremes, depending upon the number of electrical devices on the vehicle currently in use (e.g., heater, air conditioner, radio, etc.). The temperature may range from well below 0.degree.F. for a start on a cold winter morning to well over 200.degree.F. when the vehicle is operated over an extended period on a hot day. Over all of these extremes of operating conditions, the alternator should exhibit fairly good self-regulation and should accommodate rapid changes in load, temperature, and other conditions.
The alternators employed in vehicles are usually three-phase machines. The stator winding ordinarily comprises three multi-coil sections; in each winding section, each coil encircles three stator poles. The coils of each section are connected in series with adjacent coils and the coils are often separated from each other by three stator poles that are not encircled by any coil of that section.
The rotary electromagnetic excitation structures of vehicle alternators are also reasonably well standardized. Thus, the excitation structure affords a series of alternate north and south rotor poles disposed concentrically with the stator and separated from the stator poles by a small air gap. The usual practice is to taper the rotor poles so that the narrow end of each rotor pole is approximately the width of one stator pole and the wide end is approximately as wide as three stator poles. With this tapered rotor pole construction, each rotor pole covers one full stator pole and somewhat less than one-half of each of two adjacent stator poles.
Most automotive alternators utilize a complete rotary excitation structure, including an excitation coil which rotates with the rotor, so that the exciting current for the coil must be applied through brush and slip-ring circuit connections. On the other hand, automotive alternators are occasionally of so-called "brushless" construction, in which the exciting coil is stationary so that the slip-rings and brushes can be eliminated. A brushless alternator affords substantial advantages with respect to elimination of the wear and maintenance problems almost inevitably associated with brushes and slip-rings. On the other hand, the brushless construction introduces an additional air gap or gaps into the magnetic structure of the alternator, with some reduction in efficiency, a reduction that may be sufficient to offset the advantages attained by elimination of brushes and slip rings.
An excellent example of a brushless alternator construction is set forth in Barrett U.S. Pat. No. 3,493,800, issued Feb. 3, 1970. The bearing arrangement shown in that patent, particularly in the embodiment illustrated in FIG. 10, affords superior performance for a variety of applications, and particularly in alternators used in vehicles. However, the magnetic structure is not as efficient as it might be, at least for some critical applications, due to losses in the air gaps between the rotor and the stationary portion of the magnetic excitation structure.